Google SERP showing wrong regional version after redirects

Situation

Our website (approx. 3000 pages) contains multiple regional versions of the same page (eg. en-US and en-GB).
Correct href lang tags are setup in the sitemap.xml to inform google which version to serve in which region. This has worked fine.

Two weeks ago, we restructured the URLs on our website. All old URLs have proper 301 redirects.
Sitemap.xml only contains new URLs, again with proper href lang tags.

Problem

Currently, for some pages the google SERP currently shows multiple versions of the same page. This isn’t a big issue on itself, but old URLs for the wrong region are showing (higher) than the new, correct URL, which is a problem. For example, a search in UK is showing both of these results in the SERP:

site.com/old-url (en-US version = WRONG)

site.com/new-url (en-GB version = CORRECT)

It’s only 2 weeks, so I guess it will take some more time for the old URLs to disappear from the google index, but I’m not sure if time will fix this issue.

Search Console info for the old URL:

Indexed, not submitted in sitemap

User-declared canonical: site.com/new-url

Google-selected canonical: Inspected URL

I tried reindexing the old URL in Search Console. Google crawled the page again, but still chooses the Inspected URL (= the redirected URL) above the user-declared canonical (= current URL).

Possible cause

Investigating why Google doesn’t show the desired URL, I found the old URLs all seem to have backlinks.
One example has only 3 backlinks, another has 136 (too much to request fixing them).

If this is indeed the reason why Google chooses to still show the old URLs, then maybe I should focus on giving google back the correct href lang signals for the old URLs.
Would it be wise to re-add the old URL’s to the sitemap.xml so Google has correct href lang signals for these old URLs again?
I’m not keen on serving outdated URL’s to Google. , but I don’t see any other way to give google correct signals.

That is, if there's really now way to get the old URL's out of the google SERP. Maybe, I should just be more patient...?